My least used are probably....
In game: Poison, Dark, Rock and possibly Ground.
Endgame: Bug, Ice
Ice is just a rare occurrence, by the time you get to them, chances are you have formed an attachment to the pokemon you already have. :/
Same thing goes for ground, most likely, the most common ground types you'll find are the Geodude-types. (though Dugtrio in gen 1 was different, since you coudl catch one at level 28, great for Lt Surge)
Could've sworn ive seen that glyph in your avatar before Soren...maybe a character from one of the Battle Networks...